The goal of this project is to develop a commercial software package for rapid Monte Carlo-based dose computation in imaging and radiation therapy. The product takes advantage of a desktop parallel computer equipped with emerging hardware originally developed gaming and high-performance computing. The project is motivated by the fact that the market is ripe for a product that brings massively parallel computing power currently available only to National Labs and major research universities to busy and resource-limited radiology/oncology clinics. The preliminary studies by us, as well as other groups, on this very new research topic have thus far revealed a number of major challenges. Specific Aims: (Phase I Segment) 1. To develop a prototype ARCHER software/hardware system suitable for third-party user testing. 2. To demonstrate performance advantage of ARCHER against clinical radiation treatment planning system. (Phase II Segment) 1. To finish the ARCHER software design for radiation therapy dose calculations using emerging parallel-computing hardware platforms. 2. To systematically benchmark radiation physics modeling using experiments and well-tested Monte Carlo code packages. 3. To evaluate clinical benefits of ARCHER for selected radiation treatment protocols involving prostate cancer patients. 4. To evaluate software user experience for interactive and easy-to-use dose verification and treatment planning.